The present invention relates to a semiconductor device having a heterojunction portion such as a bipolar transistor, a diode, or an I2L.
Because of its excellent RF characteristics, a bipolar transistor has conventionally been used as an active device operable in the microwave/milliwave bands. In particular, most vigorous research and development has been directed to a heterojunction bipolar transistor (HBT) using a III-V compound semiconductor such as GaAs. In recent years, attention has been focused on a HBT using a SiGe material, which is a IV-IV compound material that can be fabricated on a low-cost silicon substrate.
The following are the two representative types of structures for implementing higher-speed SiGe HBTs. One of the two types is a HBT reported in Document 1 (L. Harame et al., xe2x80x9cOptimization of SiGe HBT Technology for High Speed Analog and Mixed-Signal Applications,xe2x80x9d IEDM Tech. Dig. 1993, p.71), which comprises a Si collector layer, a SiGe base layer, and a Si emitter layer. In the SiGe base layer, a Ge composition ratio is increased gradually from a region in contact with the Si emitter layer toward a region in contact with the Si collector layer to provide a graded composition base layer. The other of the two types is a HBT reported in Document 2 (A. Schuppen et al., xe2x80x9cEnhanced SiGe Heterojunction Bipolar Transistors with 160 GHz-fmax,xe2x80x9d IEDM Tech. Dig. 1995, p.743.), which comprises a Si collector layer, a SiGe base layer, and a Si emitter layer. The SiGe base layer has an extremely reduced thickness, an increased Ge composition ratio, and an increased doping concentration to provide a uniform composition base layer.
FIG. 18 is a band diagram of the former heterojunction bipolar transistor having the graded composition base layer. As can be seen from the band state shown in the drawing, an electric field induced by the graded composition causes carriers injected into the SiGe base layer to drift in the SiGe base layer toward the collector layer. Since the traveling of the carriers caused by the drift electric field is at a higher speed than the traveling thereof caused by diffusion, a base transit time is reduced and excellent RF characteristics are obtained.
FIG. 19 is a band diagram of the latter heterojunction bipolar transistor having the uniform composition base structure. As can be seen from the band state shown in the drawing, the base layer is extremely thinned to reduce the base transit time and provide excellent RF characteristics. In this case, the thinning of the base layer incurs the risk of increasing the base resistance, so that the base layer is doped with a high-concentration impurity to lower the base resistance. In addition, SiGe having a high Ge composition ratio is used in the base layer to prevent reverse injection of carriers from the base layer doped with the high-concentration impurity into the emitter, so that a heterojunction barrier formed between the SiGe base layer and the Si emitter layer is increased. In this case also, excellent RF characteristics are obtained. In particular, the doping concentration in the base layer is increased to reduce the base resistance and thereby increase a maximum oscillation frequency.
In a conventional Si LSI using a bipolar transistor, on the other hand, it has frequently been performed to compose a diode by using a PN junction portion between the base and collector of the NPN bipolar transistor and use the diode as an element of a logic circuit. This is because the NPN bipolar transistor has a structure suitable for the formation of a large number of built-in diodes since the PN junction portion between the base and collector has a high breakdown voltage (with respect to a reverse bias) and the N-type collector layer is used as a common region in the substrate.
However, such a PN junction diode has the drawback that it is unsuitable for use in a device operating at a high speed due to minority carriers accumulated in each of the P-type and N-type regions thereof. Specifically, electrons as minority carriers are accumulated in the P-type base layer of the NPN bipolar transistor layer, while holes as minority carriers are accumulated in the N-type collector layer thereof. In a typical high-speed bipolar transistor, the P-type base layer is formed extremely thin to reduce the base transit time so that the accumulation of the electrons in the P-type base layer presents substantially no problem even in the PN junction diode composed by using a part of the bipolar transistor. However, the N-type collector layer is formed to have a sufficient thickness in the range of 0.5 to 1 xcexcm in order to retain a high breakdown voltage, so that numerous holes are accumulated therein, which eventually limits the speed of the PN junction diode.
As a method of increasing the operating speed by suppressing the accumulation of minority carriers in such a collector region, there has been known one reported in Document 3 (M. Ugajin et al., xe2x80x9cThe base-collector heterojunction effect in SiGe-base bipolar transistors,xe2x80x9d Solid-State Electron., vol.34, pp.593, 1991), in which a SiGe/Si heterojunction is provided in the base/collector junction to impart a wider band gap to the collector layer. By thus imparting the wider band gap to the collector layer, a heterojunction barrier is formed in the base/collector junction portion to suppress injection of holes from the base layer to the collector layer, thereby reducing the amount of holes accumulated in the collector and increasing the operating speed of the diode.
There has also been known a method disclosed in Document 4 (M. Karlsteen et al., xe2x80x9cImproved switch time of I2L at low power consumption by using a SiGe heterojunction bipolar transistor,xe2x80x9d Solid-State Electron., vol.38, pp.1401, 1995), in which a heterojunction is provided in a base/collector junction portion in an I2L (Integrated Injection Logic) circuit into which a plurality of bipolar transistors have been integrated, thereby suppressing the accumulation of minority carriers and increasing the operating speed.
However, the aforesaid bipolar transistor and the diode using the bipolar transistor have the following disadvantages.
In the conventional HBT using the graded composition base shown in FIG. 18, it is required to greatly vary the Ge composition ratio in order to increase the intensity of the drift electric field induced by the graded composition. In short, it is required to minimize the Ge composition ratio in a region of the base layer in contact with the emitter layer and maximize the Ge composition ratio in a region of the base layer in contact with the collector layer. To satisfy the requirement, the region of the base layer in contact with the emitter layer normally has a pure Si composition without containing Ge, so that the base/emitter PN junction forms a silicon/silicon homojunction. In increasing the maximum oscillation frequency fmax of the HBT, it is effective to reduce the. base resistance as represented by the following equation (1). If a base doping concentration is increased to reduce the base resistance, however, an increased number of holes are naturally injected from the base layer into the emitter layer.
In the case where the emitter/base junction forms a homojunction or where the emitter/base junction forms a heterojunction but has a nearly pure Si composition at the end of the base, the quantity of carriers reversely injected into the emitter is increased because the base layer has no heterojunction barrier at all or, if any, an extremely low heterojunction barrier, so that the current amplification factor xcex2 is not increased.                               f          max                =                                            f              T                                      8              ⁢                              π                ·                                  R                  B                                ·                                  C                  BC                                                                                        (        1        )            
fr: current gain cutoff frequency
RB: base resistance
CBC: base/collector junction capacitance
The fact that the current amplification factor xcex2 is not increased can also be derived from the relationship represented by the following equation (2), which is established among the current amplification factor xcex2, the band discontinuity value xcex94Ev of a valence band at the emitter/base junction portion, and a carrier concentration NB in the base layer.                     β        =                                            J              n                                      J              p                                =                                    (                                                N                  E                                                  N                  B                                            )                        ·                          (                                                V                  n                                                  V                  p                                            )                        ·                          exp              ⁡                              (                                                      Δ                    ⁢                                          xe2x80x83                                        ⁢                                          E                      v                                                        kT                                )                                                                        (        2        )            
NE: carrier concentration in emitter layer
NB: carrier concentration in base layer
Vn: speed of electron diffusion in base layer
Vp: speed of hole diffusion in emitter layer
k: Boltzmann""s constant
T: absolute temperature.
In the case of using such a graded composition base, it becomes therefore possible to reduce the base transit time of carriers and improve the current gain cutoff frequency fT. However, the increase of the maximum oscillation frequency fmax cannot eventually be expected since the concentration of carriers in the base layer cannot be increased.
On the other hand, the conventional structure using the uniform composition base shown in FIG. 19 can suppress reverse injection of carriers from the base layer because a high heterojunction barrier is formed between the SiGe base layer having a high Ge composition ratio and a Si emitter layer. If the concentration of carriers in the SiGe base layer (base doping concentration) is to be further increased in order to increase the maximum oscillation frequency fmax, as described above, the quantity of carriers reversely injected is increased. To prevent this, it is required to further enhance the height of the heterojunction barrier by further increasing the Ge composition ratio in the SiGe base layer, which increases the difference in lattice constant between the emitter layer and the base layer. As a result, a critical film thickness at which a dislocation occurs in the base layer presents a problem.
It is generally known that a lattice strain develops in a SiGe layer formed by crystal growth on a Si substrate due to the difference in lattice constant between Si and Ge, which is released if the Ge composition ratio is high and the film thickness is large and thereby causes a dislocation in the film as well as a fatal damage to the element. The film thickness at which a dislocation occurs is generally termed the critical film thickness. The critical film thickness is smaller as the Ge composition ratio in the SiGe layer is higher. The critical film thickness is on the order of 50 nm when the Ge composition ratio is 30%, which corresponds to the film thickness of the base layer.
Even when the film thickness of the SiGe layer is equal to or smaller than the critical film thickness, the SiGe layer is not in a completely stable state but in a quasi-stable state if the Ge composition ratio is high. If a high-temperature process is performed in a subsequent step, such a defect as dislocation is easily caused. Hence, it is inappropriate in terms of device reliability and a thermal budget during a device fabrication process to use a SiGe layer having a high Ge composition ratio.
Therefore, there is a limit to the improvement of the RF characteristics of a bipolar transistor which is accomplished by providing a heterojunction in the emitter/base junction and enhancing the function of suppressing reverse injection of carriers from the base into the emitter through the formation of a heterojunction.
On the other hand, there is a limit to the increase of the operating speed of a conventional device such as a bipolar transistor or diode reported in Document 3 or 4 which is accomplished by providing a heterojunction in the base/collector junction, suppressing injection of minority carriers from the base layer into the collector layer, and thereby reducing the accumulated carriers.
The reason for this is that, as reported in Document 3, a considerably high heterojunction barrier is required to reduce the quantity of holes accumulated in the N-type Si collector layer of a Si/SiGe HBT to the order of the quantity of electrons accumulated in the P-type SiGe base layer thereof. Specifically, a heterojunction barrier of a height of the order of 0.2 eV is required. To form such a heterojunction barrier at the base/collector junction composed of a SiGe/Si multiple layer, the Ge composition ratio in the base layer should be at least 25% or higher.
However, since the thickness of the base layer is preferably reduced to a value equal to or smaller than the critical film thickness and device reliability and the thermal budget during the device fabrication process should be ensured, as described above, a Ge composition ratio in the SiGe layer which is 25% or higher is excessively high in terms of reliability and manufacturability.
Hence, it is problematic to increase the Ge composition ratio in the SiGe layer in order to increase the height of a heterojunction barrier formed at the base/collector heterojunction.
A first object of the present invention is to provide a bipolar transistor wherein a current amplification factor is increased with the provision of a region having the function of suppressing reverse injection of carriers from the base layer into the emitter layer irrespective of a barrier formed at the emitter/base junction. In the bipolar transistor, the current amplification factor can be improved even when a base doping concentration is increased to increase the maximum oscillation frequency fmax by relaxing restrictions on the increase of the current amplification factor.
A second object of the present invention is to provide an element functioning as a bipolar transistor, a diode, an I2L element, or the like which operates at a high speed by suppressing the injection of minority carriers from the base layer into the collector layer and thereby reducing the quantity of minority carriers accumulated in the collector layer. This can be achieved by providing means for effectively increasing a heterojunction barrier formed at the base/collector junction without increasing the Ge composition ratio in the SiGe base layer to such a value as to excessively reduce the critical film thickness, i.e., by implementing a structure which ensures sufficiently high reliability.
To attain the first object, a first bipolar transistor according to the present invention has a multi-quantum barrier (MQB) composed of a superlattice structure consisting of two types of extremely thin films having different compositions and alternately stacked, which is provided in a region of the emitter adjacent the emitter/base junction. The MQB reflects a wave of carriers reversely injected from the base, which fact is used to effectively increase the height of a hetero-junction barrier (barrier height) and thereby suppress the reverse injection of the carriers from the base layer. Specifically, the first bipolar transistor has the following structure.
The first bipolar transistor according to the present invention comprises an emitter layer, a base layer, and a collector layer, the bipolar transistor having a multi-quantum barrier portion being provided in the emitter layer and composed of a plurality of barrier layers and well layers alternately stacked to perform the function of reflecting an incident wave of minority carriers in the emitter layer injected from the base layer and provide such a phase that the incident wave of carriers and a reflected wave of carriers intensify each other.
With the arrangement, not only a barrier induced by the discontinued valence band at the emitter/base junction but also the reflecting function of the multi-quantum barrier portion prevents reverse injection of carriers from the base layer. Since the reverse injection of carriers is suppressed, it becomes possible to increase the current amplification factor and improve RF characteristics including maximum oscillation frequency fmax even if the carrier concentration in the base layer is increased.
By composing the barrier layers and well layers of the multi-quantum barrier portion of respective semiconductor materials having different band gaps, there can easily be implemented a multi-quantum barrier layer having the function of suppressing reverse injection of carriers.
By setting the band discontinuity value of a band in the emitter layer containing the multi-quantum barrier portion, through which majority carriers flow, at a substantially negligible value, there is provided a band structure presenting no obstacle to the movement of majority carriers in the emitter layer, which enhances the effect of improving a current amplification factor.
With the base layer being strained, a particularly high effect is achieved when the difference in lattice constant between the emitter layer and the base layer is large.
By composing the emitter layer and the base layer of respective semiconductor materials having different band gaps and providing, in a band gap between a valence band and a conduction band in the base layer, a portion gradually decreasing from a region of the base layer in contact with the emitter layer toward a region of the base layer in contact with the collector layer, the traveling speed of carriers in the base layer is determined by a drift velocity, not by a diffusion speed, so that the base transit time is reduced and a current gain cut-off frequency fT is increased. In addition, the multi-quantum barrier portion. suppresses reverse junction of carriers from the base layer into the emitter layer irrespective of the band discontinuity value reduced by the provision of the graded composition base. Moreover, there can also be achieved a reduced base resistance attributable to higher-concentration base doping or an increased thickness of the base layer, which increases a maximum oscillation frequency fmax.
By composing the multi-quantum barrier portion of a superlattice structure of a Silxe2x88x92xxe2x88x92yGexCy/Si multiple layer, the critical film thickness of the Silxe2x88x92xxe2x88x92yGexCy layer at the multi-quantum barrier portion is particularly increased, so that it becomes possible to further increase the effective barrier height of the multi-quantum barrier portion without incurring a dislocation.
With the multi-quantum barrier portion being disposed in a region of the emitter layer exterior to a depletion region formed between the emitter layer and the base layer at a working voltage when the transistor is operating, the function of suppressing reverse injection of carriers from the base into the emitter can maximally be performed in any operating condition.
By disposing the barrier layer at the end of the multi-quantum barrier portion closer to the base layer in such a position as to prevent the tunneling of the carriers from the depletion region formed between the emitter layer and the base layer to the well layer adjacent to the barrier layer at the end of the multi-quantum barrier portion closer to the base layer, such improved RF characteristics as described above can be expected without degrading the function of suppressing reverse injection of carriers performed by the multi-quantum barrier portion.
To attain the first object, a second bipolar according to the present invention has a high-concentration doped layer for effectively enhancing the function of suppressing reverse injection of carriers from the base layer into the emitter layer, which is provided in the emitter layer of the bipolar transistor. Specifically, the second bipolar has the following structure.
The second bipolar transistor according to the present invention comprises an emitter layer containing an impurity of a first conductivity type, a base layer containing an impurity of a second conductivity type, and a collector layer containing the impurity of the first conductivity type, the bipolar transistor having a high-concentration doped layer being provided in the emitter layer and doped with the impurity of the first conductivity type at a higher concentration than in the emitter layer.
This produces not only a barrier induced by a discontinued valence band at the emitter/base junction but also a potential barrier induced by a high-concentration doped layer in the valence band of the emitter layer, so that the carriers in the base layer are inhibited from being reversely injected into the emitter layer. By suppressing reverse injection of carriers, therefore, it becomes possible to improve the current amplification factor as well as the RF characteristics including maximum oscillation frequency fmax even if the carrier concentration in the base layer is increased.
Preferably, the high-concentration doped layer is a xcex4-doped layer having a thickness of 10 nm or less.
Preferably, the concentration of carriers of the first conductivity type in the high-concentration doped layer is 1xc3x971019 cmxe2x88x923 or more.
Preferably, the concentration of carriers of the first conductivity type in the high-concentration doped layer is more than ten times higher than the concentration of the carriers of the first conductivity type in the emitter layer.
With the high-concentration doped layer being adjacent to a depletion region formed at an emitter/base junction portion, there can maximally be performed the function of suppressing reverse injection of carriers from the base to the emitter.
Preferably, the concentration of carriers of the second conductivity type in the base layer is higher than the concentration of carriers of the first conductivity type in the emitter layer.
By composing the emitter layer and the base layer of two types of semiconductor materials having different band gaps, imparting the wider band gap to the semiconductor material composing the emitter layer, and providing a heterojunction portion between the emitter layer and the base layer, the function of suppressing reverse injection of carriers from the base layer is further enhanced by using a high barrier at the heterojunction portion. In that case, the strained base layer achieves a particularly high effect when the difference in lattice constant between the emitter layer and the base layer is large.
To attain the second object, a semiconductor device according to the present invention has a multi-quantum barrier (MQB) having a superlattice structure consisting of two types of extremely thin films having different compositions and alternately stacked, which is provided in a region of the collector layer in contact with the collector/base junction. The height of the heterojunction barrier (barrier height) has been effectively increased by using the effect of reflecting a wave of carriers injected from the base (minority carriers in the collector layer). Specifically, the semiconductor device has the following structure.
The semiconductor device according to the present invention comprises a bipolar transistor comprises an emitter layer, a base layer, and a collector layer, the bipolar transistor having a multi-quantum barrier portion being provided in the collector layer and composed of a plurality of barrier layers and well layers alternately stacked to perform the function of reflecting an incident wave of minority carriers in the collector layer injected from the base layer (minority carriers in the collector layer) and provide such a phase that the incident wave and a reflected wave intensify each other.
In the arrangement, the carriers in the base layer are prevented from being injected into the collector layer not only by a barrier induced by a discontinued valance band at the collector/base junction but also by the reflecting function performed by the multi-quantum barrier portion. By suppressing the injection of minority carriers, therefore, the accumulation of the minority carriers in the collector layer is prevented and the operating speed of a bipolar transistor or the like is increased even if the carrier concentration in the base layer is increased.
The semiconductor device can also comprise an additional structure similar to the structure of the first bipolar.
By further providing the semiconductor device with an element having, as components, two regions having the same structure as a base/collector junction of the bipolar transistor, there can be obtained an element operating at a high speed by using the function of suppressing injection of minority carriers performed by the multi-quantum barrier portion. The element is, e.g., a diode.
By further providing the semiconductor device with another bipolar transistor comprising a collector layer disposed in a region used commonly by the base layer of the bipolar transistor, a base layer disposed in a region used commonly by the collector layer of the bipolar transistor, and an emitter layer, the semiconductor device is caused to function as an I2L element occupying a smaller area and operating at a high speed.
In that case, it is also possible to provide at least one other collector layer connected to the base layer of the bipolar transistor and another multi-quantum barrier portion being disposed in the one other collector layer and composed of a plurality of barrier layers and well layers alternately stacked to perform the function of reflecting an incident wave of carriers injected from the base layer (minority carriers in the collector layer) and provide such a phase that the incident wave and a reflected wave intensify each other.